1. Field of the Invention
The present invention relates generally to safety valves used in oil and gas wellbores and, more particularly, to a torsional safety valve mechanism used to affix and maintain the flapper in a closed position.
2. Description of the Related Art
The use of safety valves in oil and gas wellbores is well known in the art. In general, these valves are used as redundant shut-offs in the event of wellbore emergency. These valves typically consist of a flapper which is pivotally connected to a tubular member via a hinge. During fluid production, a sleeve is generally used to actuate the flapper in an open position, allowing fluid to pass through. However, in the event of an emergency or other event, the flapper can be closed, preventing dangerous fluids from traveling up hole to the surface and being released into the environment.
In the past, various manufacturers have used steel pins, called flapper pins, to connect the flapper to the hinge on a mating part of the tubular member called a hard seat. Once the flapper is placed on the hinge, a set of radially contoured pins are placed on either side of the flapper to support the torsion springs. The torsion springs circumferentially wrap around the pins and are used to bias the flapper in a closed position. Tabs extend from the torsion springs out over the flapper, contacting the flapper, thereby transferring the spring torque to the flapper and biasing it in the closed position.
There are disadvantages to the traditional flapper mechanism design. First, since the tabs form an integral part of the springs, the torque stresses created by the spring's movement when the flapper is opened is transferred along the spring itself, increasing the likelihood of stress fractures and spring failure. The torque stresses are typically the greatest at the location where the tab and spring portions join. Second, when using a contoured flapper, the tabs extending from the springs often slide along or off the flapper surface when it is actuated in the opened positioned. This decreases valve efficiency and creates additional stresses on the spring, which results in combined loading and increased fatigue stresses along the spring and flapper surface. In those designs where the tabs connect with each other over the flapper surface, even more fatigue stress is created at the tab/spring junction. Third, the use of the individual flapper pin is unduly complicated and increases the cost associated with the safety valve.
In view of these disadvantages, there is a need in the art for an improved flapper mechanism which minimizes the torque stresses on the torsion springs and simplifies the flapper design, thereby providing a more reliable and cost-efficient safety valve.